1. Field of the Invention
The present invention relates to a heater controlling apparatus for controlling a gas sensor heater of a gas sensor, which heats a sensing element of a gas sensor provided in an exhaust system of a diesel engine.
2. Description of Related Art
Due to strengthening a regulation of exhaust gas, an air fuel ratio control in a vehicle engine has become more important. Thus, in order to perform a feedback control of the air fuel ratio, a gas sensor is provided in an exhaust gas system of the engine to measure an oxygen concentration in the exhaust gas. When the gas sensor is used, a sensing element needs to be heated to an appropriate temperature by a heater in order to activate the sensing element. However, when the sensing element is heated by energizing the heater while water exists in the exhaust system (exhaust pipe) upstream of the gas sensor, the water may attach a surface of the sensing element and the element may be disadvantageously broken.
Thus, in general, a temperature of a wall of the exhaust pipe is assumed based on the exhaust temperature and on an engine coolant temperature, and then it is determined whether the water exists or not based on the wall temperature. When the wall temperature becomes a temperature, at which a heater energization can be made, the heater energization is started (see Japanese Unexamined Patent Publication 2003-328821 corresponding to U.S. Pat. No. 6,781,098).
In contrast, the diesel engine features in having a low exhaust gas temperature due to its high thermal efficiency. Therefore, for example, when the engine is stopped after a preceding operational state, where the exhaust temperature is kept low (e.g., the temperature in the exhaust pipe is kept equal to or less than 50° C. in an idling operation), condensed water may be accumulated (left) in the exhaust system because of the continuous state of the low exhaust temperature.
However, a conventional heater controlling apparatus determines timing for the heater energization only based on limited information (e.g., the wall temperature of the exhaust pipe) when a following engine operation begins, without considering an influence due to the condensed water accumulated in the preceding engine operation. In this case, the heater energization is performed in a condition, where the condensed water still exists in the exhaust system. Thus, water drop from the above condensed water may be scattered toward the heated sensing element. As a result, the sensing element may be exposed to the water and may be disadvantageously broken.
Also, it may be proposed that the entire heater energization may be delayed in order to limit breakage of the sensing element due to the exposure to the water. However, unnecessary delay of the heater energization may limit the gas sensor from being quickly activated, and therefore the unnecessary delay may become one cause for limiting purification efficiency of the exhaust gas.